docs/library/network.WLAN: Add "txpower" to list of known WLAN configs.

Signed-off-by: Damien George <damien@micropython.org>

docs/library/network.WLAN.rst

@@ -132,4 +132,5 @@ Methods
    password       Access password (string)
    dhcp_hostname  The DHCP hostname to use
    reconnects     Number of reconnect attempts to make (integer, 0=none, -1=unlimited)
+   txpower        Maximum transmit power in dBm (integer or float)
    =============  ===========